title: Optical birefringence studies of a binary mixture with the nematic-smectic Ad-re-entrant nematic phase sequence.
abstract: We report the measurements of birefringence as a function of temperature of a binary system 4-cyanophenyl [4'(4''-n-heptylphenyl)]benzoate (7CPB) + 4-cyanophenyl 4-nonylbenzoate (9.CN) showing a nematic-smectic A(d)-re-entrant nematic phase sequence by means of the optical transmission method. The temperature dependence of the birefringence has been determined from the transmitted intensity data and the orientational order parameters have been calculated. These observations indicate that re-entrant nematic to induced smectic A(d) and induced smectic A(d) to nematic phase transitions for all the mixtures are of second order. There is a continuous change in the Δn values at the nematic-smectic A(d) and smectic A(d)-re-entrant nematic phase transitions. However, for some mixtures a slight increase in birefringence on cooling in the vicinity of the smectic A(d)-re-entrant nematic transition has been observed. We have also fitted our experimental results with those calculated from the modified McMillan theory as proposed by Luckhurst and Timimi.
authors.affiliation: Department of Physics, North Bengal University, Siliguri, India.
authors.forename: Akhileshwar
authors.initials: A
authors.lastname: Prasad
authors.forename: Malay Kumar
authors.initials: MK
authors.lastname: Das
mesh_terms: D001718:Birefringence; D050866:Liquid Crystals / Q000737:chemistry*; D044367:Phase Transition